(A) Field of the Invention
This invention relates to a process for the selective alkylation of the unsubstituted ortho position or positions of phenolic compounds. More particularly, it relates to a process for the selective ortho-alkylation of a phenolic compound by reacting the phenolic compound with an alcohol in the vapor phase in the presence of a catalyst containing mixed oxides of chromium oxide and tin oxide.
(B) Description of the Prior Art
The preparation of 2,6-dimethylphenol, among other ortho-alkylated phenols, has heretofore been the subject of many studies it is useful as a raw material for the manufacture of polyphenyleneoxide having a wide range of utility in the field of synthetic resins.
Currently, a process for the ortho-alkylation of phenols is in industrial use which involves the vapor phase reaction of a phenol with an alcohol in the presence of an acidic solid catalyst such as alumina. However, in this process, the selectivity in the site of alkylation is only limited. That is, not only the ortho positions of the phenolic nucleus but also the meta and para positions thereof are subject to alkylation, so that a complicated procedure for the separation and purification of desired reaction products if required.
Another industrial process for the ortho-alkylation of phenols is based on vapor phase reaction in the presence of a magnesium oxide catalyst. However, this catalyst has inherently low temperature activity and requires high temperatures of 475.degree. C. or higher, practically 500.degree. C. or higher, to achieve sufficient reaction. Moreover, its activity tends to become lower during a short period time of reaction.
In order to solve these problems, there have been proposed a number of catalysts including, for example, those comprising various combinations of magnesium oxide and another oxides, those comprising various combinations of iron oxide and other oxides, and the like. However, these catalysts still have the disadvantages that they are insufficient in selectivity for ortho-alkylation and in service life of the catalysts and that polyalkylated products such as 2,4,6-trialkylphenols are formed under reaction contitions suited for the enhancement of their insufficient activity. In addition, a chromium-iron mixed oxide catalyst is disclosed in Japanese Patent Publication No. 12610/'76. However, when phenol and methanol are used as starting materials, the yield of the desired product e.g. 2,6-dimethylphenol, is as low as about 40%. This makes the catalyst useless for practical purposes.